1. Field of the Invention
This invention generally relates to a photoelectric device for converting light energy into electrical energy, and, in particular, to an amorphous silicon photoelectric device which uses amorphous silicon for providing an electrical signal in response to light radiation.
2. Description of the Prior Art
Photoelectric devices have been used in solar cells and image sensors, and some of them utilize single crystalline silicon and others utilize lately developed amorphous silicon as a material for converting light energy into electrical energy. In either case, the photoelectric device is structured to contain either a p-n or p-i-n junction, which serves to convert light energy into electrical energy.
In the case of a photoelectric device having either a p-n or p-i-n junction using single crystalline silicon, the efficiency of converting light energy into electrical energy is high, typically approximately 15%, but since use is made of single crystalline silicon as a material, there are such disadvantages as, relatively high manufacturing cost and difficulty in manufacturing a device having a relatively large surface area. On the other hand, in the case of a photoelectric device comprised of amorphous silicon, the manufacturing cost is significantly lower and a device having a large surface area can be manufactured with ease. It is to be worth noting that the photoelectric conversion efficiency of a recent device using amorphous silicon is 10% or more, and this efficiency is expected to increase more.
Typical prior art photoelectric devices using amorphous silicon, or simply a-Si, are shown in cross section in FIGS. 1 and 2. The prior art photoelectric device shown in FIG. 1 includes a light-transmitting substrate 1, such as glass, a transparent, electrically conductive film 2, such as ITO or SnO.sub.2, p.sup.+ type a-Si:H(B) film 4, an i type a-Si:H film 4, an n.sup.+ type a-Si:H(P) film 5, and a metal electrode film 6. Such a prior art photoelectric device is disclosed, for example, by D. E. Carlson in the Journal of Electric Materials, Vol. 6, No. 2, 1977, which is hereby incorporated by reference. The other prior art structure shown in FIG. 2 is basically similar to the structure of FIG. 1; however, in the structure of FIG. 1, other atoms than B are not added to the p type layer, but, in the structure of FIG. 2, in the p.sup.+ type a-Si film 7 is added C, O, or N atoms other than B so as to make it a wide band-gap film having the so-called "window effect." In this respect, p.sup.+ type a-Si:C:H(B) is disclosed in Japanese Pat. Laid-open Pub. Nos. 56-64476 and 57-181176, p.sup.+ type a-Si:N:H(B) is disclosed in Japanese Pat. Laid-open Pub. No. 56-96878, and p.sup.+ type a-Si:O:H(B) is disclosed in Japanese Pat. Laid-open Pub. No. 56-142680.
In the photoelectric device shown in FIG. 1, it is said that the better conversion efficiency is obtained if light is irradiated in the direction indicated by the arrow. This is because, B atoms added to the p.sup.+ type layer are diffused into the i type layer, thereby providing a p-i interface effective in photoelectric conversion. When light is irradiated as shown in FIG. 1, since the p type layer does not have a photoconductive characteristic, the generation of photo-carriers takes place in the i type layer. In this case, the light transmission efficiency of the p type layer determines the efficiency of generation of photo-carriers in the i type layer, so that in the visible light range, the p type layer is required to have a wider optical band gap. However, since the optical band gap of i type a-Si normally ranges between 1.7 and 1.8 eV and the optical band gap of p.sup.+ type a-Si ranges between 1.4 and 1.5 eV under optimal photoelectric conversion condition, 40 to 60% of the incident light is absorbed in the p type layer by recombination or trapping without generation of photo-carriers, so that the efficiency of photoelectric conversion tends to be lower.
The structure of the other prior art device shown in FIG. 2 will now be described in detail. In the first place, it is assumed that use is made of a-Si:C:H(B). In this case, manufacture of a film of a-Si:C:H is difficult. When manufacturing a film of a-Si:C:H small in local state density, it is often said to be preferable to use the plasma CVD method and a combination of CH.sub.4 and SiH.sub.4 as a start gas. However, since there is a discrepancy in decomposition rate between SiH.sub.4 and CH.sub.4, there is a difficulty in manufacture. In addition the resulting film tends to present difficulty in being etched, which thus hinders the formation of a complicated, fine pattern. Besides, similarly with a silicon atom, since a carbon atom is a group IV element, it tends to form a dangling bond and the tolerance in manufacturing films of the same quality if rather limited, so that there is a necessity to control the manufacturing process carefully.
On the other hand, in the case where use is made of a film of a-Si:N:H(B), there is a disadvantage of inability to broaden the optical band gap in the photoconductive region (cf. J.J.A.P., Vol. 21, No. 8, 1982). Thus, if this film is used as the p type film of the p-i-n device, there is obtained a result similar to the one in the case of FIG. 1. In addition, in the case of manufacturing a film of a-Si:N:H small in local state density, it is said to be preferable to use the plasma CVD method and a combination of N.sub.2 and SiH.sub.4 as a start gas; however, since the decomposition rate of N.sub.2 is inferior, there is a need to increase the plasma density, so that there is a disadvantage of deteriorating the characteristic of the resulting photoelectric device due to plasma damage.
As disclosed in Japanese Pat. Laid-open Pub. No. 56-142680, use may be made of a film of a-Si:O:H(B). In this case, a combination of O.sub.2 and SiH.sub.4 is used as a start gas; however, since SiH.sub.4 and O.sub.2 react momentarily, it is considered that voids of SiO.sub.2 added with oxygen atoms at high density are present in the resulting film. As a result, if a small quantity of oxygen atoms is added, the optical band gap is not broadened; whereas, the addition of a large quantity of oxygen atoms allows to broaden the optical band gap. However, since a large amount of voids will be present when the optical band gap is broadened by the addition of a large quantity of oxygen atoms, there will be an increase of local state density within the film, which then deteriorates the photoconductive characteristic. In the above-raised Pat. Laid-open Publication, the characteristic of a p-i-n type device is disclosed as an example, but it is used in the range of optical band gap between 1.8 and 1.9 eV, and, thus, it indicates the difficulty in using the device with the optical band gap at 2.0 eV or more.